Go Kart Chassis engineering: flexibility

[FLAVFAST]

Hi guys,

I'm building a fleet of go-kart for our local leisure track. The design include a tubular frame made from AISI 4130 chro-moly pipes (stress relieved), TIG welded with S70B2 filler metal, OD 32mm, WT 2.1mm.

After having testing the few first frames, I can say they are way too stiff! Why? Simply cutting some pipes dramatically improve handling. Playing with caster angles, track width, tire pressures does NOT have any real impact.

But here it comes: I cannot afford to modify the design of the chassis due to all others components fixed to if.

Do you have any ideas on how can I get more flex? Maybe downsize pipes diameter? Do you think I can get more flexibility on welding seams?

Thanks in advance for your help.

[flynfrog]

Buy an already made frame. There is 50+ years R&D in a modern kart frame. You can buy a used one for next to nothing. If you dont understand the why behind a modern kart frame you cant hope to improve on it.

[FLAVFAST]

Thanks flynfrog.

my design is quite inspred from existing chassis of famous brand we own for many years already.
We just find out, that by completely cutting some pipes on our new chassis, handling comes close to the old others. Ok, it's just the evidence that the issue is about flexibility. But I can't figure out why similar design and identical pipe grade / dimensions gives such different results in term of flexibility.

Any experience / opinions about heat treatment? I've seen that some frames are heat treated and stress relived but I heat treatment does not impact Youn's modulus ("springiness") of the material.

[flynfrog]

Kart frames will use different wall thickness tubes through the chassis to give it the flex you are looking for. They will also have removable tubes to help tune the chassis. Your weld type and fitup can also have a rather large effect on stiffness. Have you tried using a beam analysis software such as grape to get an idea of what your chassis is doing?

[Blanchimont]

What about the rear axle, can you change the diameter or wall thickness of it?
What about the thickness of front "uprights"?

Is the whole chassis to stiff or do you think it is just the front/rear?

[FLAVFAST]

thaks for your help.
I've been using a beam analysis software. Very usefull when you want to make comparative studdy but very time consuming when you want to reproduce the exact reality. Comparative studies shows without surprises: remove extra pipes or downsize your pipes.

I'm now planning to get softer rear axles.

[garygph]

There are so many things that can have an impact as we all know and a lot has been mentioned, however I have not seen anything about the seat. How is it secured to the chassis? Removing or adding stays is a tuning tool normally as it can be a (forgotten) stressed member of the chassis. Seat design also has an impact as I learned the hard way. I had cracked my ribs in a prior off and was impatient to drive again and anyone who has karted knows that cracked ribs and karting do not go together. So my friend/sponsor made me a seat that supported me at shoulder height and not just around the ribs. Well I had a horrible handling kart that I could not fix until the day I put the old seat back in. Massive change..not a small one. Competitive karting guys have very specific lay up requirements for their seats for the above reasons.

[Tim.Wright]

Have you tried measuring the torsional and bending stiffness of your kart vs you reference one? I think you will learn more from that than any computer analysis.

[FLAVFAST]

There are so many things that can have an impact as we all know and a lot has been mentioned, however I have not seen anything about the seat. How is it secured to the chassis? Removing or adding stays is a tuning tool normally as it can be a (forgotten) stressed member of the chassis. Seat design also has an impact as I learned the hard way. I had cracked my ribs in a prior off and was impatient to drive again and anyone who has karted knows that cracked ribs and karting do not go together. So my friend/sponsor made me a seat that supported me at shoulder height and not just around the ribs. Well I had a horrible handling kart that I could not fix until the day I put the old seat back in. Massive change..not a small one. Competitive karting guys have very specific lay up requirements for their seats for the above reasons.

Don't you think it may be mainly due to the rise of the center of gravity? We have sliding seat with arround 20cm stroke. Once again, no real changes depending of our seat position.

[FLAVFAST]

Have you tried measuring the torsional and bending stiffness of your kart vs you reference one? I think you will learn more from that than any computer analysis.

We used to measure the torsional stiffness, using 4 calibrated and aligned scales and inserting shims on the front right tire.
For the same front and rear track width, our chassis cross weight varies by 1.2% per mm. Our reference, 0.98% per mm.

How would you measure bending stiffness?

[flynfrog]

I used some dial gauges, a few clamps, a steel pipe, two bathroom scales and a pair strong gentlemen

[flynfrog]

There are so many things that can have an impact as we all know and a lot has been mentioned, however I have not seen anything about the seat. How is it secured to the chassis? Removing or adding stays is a tuning tool normally as it can be a (forgotten) stressed member of the chassis. Seat design also has an impact as I learned the hard way. I had cracked my ribs in a prior off and was impatient to drive again and anyone who has karted knows that cracked ribs and karting do not go together. So my friend/sponsor made me a seat that supported me at shoulder height and not just around the ribs. Well I had a horrible handling kart that I could not fix until the day I put the old seat back in. Massive change..not a small one. Competitive karting guys have very specific lay up requirements for their seats for the above reasons.

Don't you think it may be mainly due to the rise of the center of gravity? We have sliding seat with arround 20cm stroke. Once again, no real changes depending of our seat position.

check out Ekartinnews.com they have some pretty good people on setup. As noted earlier seat position and number of stays are a major tuning tool.